1. Field of the Invention
This invention relates to multi-phase power systems and, more particularly, to apparatus for sensing an overcurrent on one phase of a multi-phase system.
2. Description of the Prior Art
Alternating current, solid state, remote power controllers are apparatus designed for solid state switching of power circuits, such as in aerospace applications, where a high degree of compactness as well as reliability are prime goals. To achieve compactness and to permit a high degree of miniaturization, large discrete components, such as transformers, are to be avoided. Hence, it has been recognized that current transformers are not an acceptable means of sensing line current for overcurrent protection in AC power controllers of this type. In U.S. Pat. No. 3,811,073 by Shuey and Baker, issued May 14, 1974, is presented an alternating current sensing circuit and method that avoids transformers and utilizes small resistive shunts in each leg of inverse parallel thyristor power switches to produce a small full-wave rectified voltage that is proportional to load current. This voltage may be processed by an operational amplifier to produce a desired overload trip circuit or may be used to drive opto-isolator light emitting diodes to achieve an overcurrent circuit with isolation from the AC line. Such current sensing circuits are satisfactory, particularly for single phase systems. On multi-phase, such as three-phase, systems some further means has to be found in order to provide an overcurrent trip signal with similar trip times for single-phase conditions and three-phase conditions.
The above referred to patent also gives additional general background information relating to alternating current remote power controllers of the general type to which the invention is directed.